Federal regulators twice passed on investigating faulty ignition switches in GM cars that led to at least 13 deaths and more than 2.6 million recalled vehicles, a new congressional report says.

WASHINGTON — Federal regulators twice declined to investigate faulty ignition switches in General Motors Co. cars that led to 13 deaths — even though one official found "a pattern" of problems, according to a new congressional report.

The report, released Sunday, added fresh details to a controversy that has shaken the revitalized automaker.

Already under fire for lengthy delays in recalling the vehicles, GM also was accused in the report of allowing the defective part to be installed in millions of vehicles after testing showed it did not meet the company's own specifications.

The developments came in a 16-page report that summarized an investigation by the House Energy and Commerce Committee ahead of a high-profile hearing Tuesday.

"We now know the problems persisted over a decade, the red flags were many, and yet those responsible failed to connect the dots," said Rep. Fred Upton (R-Mich.), the committee's chairman.

GM Chief Executive Mary Barra and David J. Friedman, the acting administrator of the National Highway Traffic Safety Administration, are scheduled to testify Tuesday before the panel's Oversight and Investigations subcommittee about the reason for delays in recalling vehicles with the faulty ignition switch.

The defective switch caused vehicle engines to turn off, disabling the air bags. The part has been linked to a series of crashes and, by GM's latest disclosure Friday, at least 13 deaths in six models. The Detroit automaker has recalled more than 2.6 million vehicles globally.

"The revelation that NHTSA had teed up an investigation and deep-sixed it is very troubling," said Sean Kane, president of Safety Research & Strategies, an independent, for-profit safety research company.

NHTSA on Sunday defended its handling of the GM ignition switch problems.

"As we have stated previously, the agency reviewed data from a number of sources in 2007, but the data we had available at the time did not warrant a formal investigation," the agency said.

"Recent data presented by GM provides new information and evidence directly linking the ignition switch to the air bag non-deployment. That's why we are aggressively investigating the timing of GM's recall."

Meanwhile, a group of siblings and parents of 16 people who were killed or injured in crashes linked to the faulty switches have sent an open letter to Barra, seeking a meeting when she is in Washington.

"Now that the truth has come to light — with more head-shaking disclosures every day — you must do what's right," lawyer Bob Hilliard wrote on behalf of the families in a letter over the weekend.

"They need to hear from you, listen to your voice to know you are truly sorry and that you share in their grief and, to an extent at least, you understand their loss," he wrote.

Greg Martin, a GM spokesman, reiterated that company officials "deeply regret the circumstances that led to the recall" and would cooperate fully with NHTSA and Congress "to help get them a better understanding of the facts."

House investigators have received more than 235,000 pages of documents from GM and NHTSA as part of the investigation, and "they paint an unsettling picture," said Rep. Tim Murphy (R-Pa.), the subcommittee's chairman.

NHTSA and the Justice Department have opened investigations into why it took so long for GM to recall the vehicles. Documents filed with NHTSA indicate the company knew about the problem as early as 2001.

Kane said he wasn't surprised by the agency's failure to launch an investigation.

"NHTSA is very scattershot about what they decide to do and when they decide to do it, and they have resisted setting guidelines for when to decide to take enforcement actions," he said.

The chronology investigators put together demonstrated why the safety agency needs clearly established and transparent standards for launching probes and establishing recalls, he said.

After some crashes involving GM vehicles in which air bags did not deploy, the chief of NHTSA's Defects Assessment Division emailed other agency officials in September 2007 proposing an investigation into problems in 2003 to 06 Chevrolet Cobalts and Saturn Ions.

The email said there was "a pattern of reported non-deployments" of air bags in complaints from vehicle owners that first was observed in early 2005.

At the time, GM indicated "they see no specific problem pattern," the committee report said.

A PowerPoint presentation prepared by the Defects Assessment Division, dated Nov. 17, 2007, said its review of the problem was prompted by 29 complaints and four fatal crashes.

But NHTSA's Office of Defects Investigation declined to investigate further, the committee report said, noting that "the panel did not identify any discernible trend and decided not to pursue a more formal investigation."